1. Field of the Invention
The invention relates to an auxiliary or additional airfoil for aircraft main wings, in which the additional airfoil is articulated to the main wing and can be extended.
2. Background of the Related Art
In order to increase lift during a landing approach at a reduced flying speed, additional airfoils, for example leading-edge slats and landing flaps, are fitted to aircraft main wings. These additional airfoils are extended during the landing approach, so that the effective profile curvature and lifting area of the main wing are increased. Commercial aircraft generally use leading-edge slats of the xe2x80x9cHandley Page slatxe2x80x9d type and so-called xe2x80x9cFowler flapsxe2x80x9d as the landing flaps.
A trial with leading-edge slats in which the rear faces of the leading-edge slats have various shapes is described in van den Berg, B.: xe2x80x9cAn Experimental Investigation on an Airfoil with a Slat into the Effect of the Separation Bubble in the Slat Cove,xe2x80x9d NLR Memorandum AL-85-001 U, February 1985. In the xe2x80x9cbasic slat configuration,xe2x80x9d the cross-sectional shape of the leading-edge slat is designed such that the contour of the rear face of the leading-edge slat represents the negative image of the contour of the main wing leading edge. When the leading-edge slat is retracted during cruise flight, the leading-edge slat thus rests flush against the main wing.
With another cross-sectional shape, referred to as xe2x80x9cslat hook rounded off,xe2x80x9d the lower area of the leading-edge slat rear face is rounded off and does not rest flush against the main wing when the leading-edge slat is retracted.
Another cross-sectional shape, the so-called xe2x80x9cfairing in slat covexe2x80x9d has a raised area in the lower area of the leading-edge slat rear face, so that the cross-sectional shape of the leading-edge slat has a bubble contour. With this cross-sectional shape, the lift is increased when the leading-edge slat has been extended, since higher flow speeds occur in the slat cove flow between the leading-edge slat and the main wing.
DE-A 31 14 143 A1 discloses a wing having a leading-edge slat and a main wing, in which an auxiliary flap is provided on the underneath of the extendable leading-edge slat, in order to cover the slat cove between the leading-edge slat and the main wing. The lower face of the auxiliary flap is shaped such that the leading-edge slat rests flush against the main wing when it is retracted. This shape, which is matched to the main wing contour, cannot follow the shape of the separation flow line and thus cannot prevent flow detachment on the rear face of the leading-edge slat. The desired noise reduction thus cannot be achieved.
DE-A 1 907 710, DE-A 1 481 580, EP 0 227 643 A2, EP 0 188 823 A1 and U.S. Pat. No. 3,486,720 disclose corresponding wing structures in which a tab extending in the direction of the main wing is arranged on the lower face of the leading-edge slat. When retracted, this tab forms a flush termination for the leading-edge slat against the main wing. However, disadvantageously, this leads to increased noise emission.
U.S. Pat. No. 3,203,647 discloses a balloon on the leading edge of the additional airfoil of a wing structure. When the additional airfoil is extended, this balloon can be inflated, thus avoiding flow separation on the normally sharp leading edge of the additional airfoil.
Based on above knowledge and the known wing combinations, an object of the invention is to reduce the noise emission from commercial aircraft during landing approaches. During landing approaches, the noise from flow passing around the wing and the fuselage is generally greater than the noise from the engines, since engine noise reduction measures have already been successfully implemented for landing approaches. The most important aerodynamic noise sources are the landing gear, the leading-edge slats and the side edges of the landing flaps.
The above and other objects are achieved by an additional airfoil for an aircraft main wing, wherein a separating surface is arranged on the additional airfoil. The separating surface extends in the direction of the main wing, and is arranged along a separation flow line between a vortex flow region and a slat cove flow of the air flowing between the additional airfoil and the main wing.
When extended, flow detachment of the slat cove flow between the leading-edge slat and the main wing occurs in the form of a pronounced vortex on the rear face of the leading-edge slat. This vortex is continuously supplied with new energy from the adjacent slat cove flow. It has been found that balls of turbulence continuously enter the accelerated slat cove flow via the separation flow line between the vortex flow region and the slat cove flow, thus resulting in noise being produced. Furthermore, it has been found that noise is also produced by the balls of turbulence flowing away over the trailing edge of the leading-edge slat. In addition, it has been found that the geometric position of the vortex is not stable, so that the oscillating position of the vortex axis results in superimposition of an additional unsteady speed component, likewise resulting in aerodynamic noise being produced.
It has been found that these above-mentioned flow sources can be minimized by fitting a separating surface along the separation flow line between the vortex and the slat cove flow, without influencing the lift. This separating surface prevents impulse exchange transversely with respect to the direction of the slat cove flow.
It is sufficient for the vortex region to be only partially covered and for the separating surface not to enclose the entire rearward leading-edge slat profile. This allows the aerodynamic noise of the entire aircraft to be reduced by about 3 dB.
According to one embodiment, the separating surface may be rigid and in this case is preferably hinged on the additional airfoil such that it is capable of being pivoted in or retracted when the additional airfoil is retracted in the direction of the main wing. This ensures that, during cruise flight, the leading-edge slat can be positioned flush against the main wing, and no significant aerodynamic losses occur.
In accordance with another embodiment, the separating surface may also be flexible and can be retracted, e.g., by being pushed into the cove between the leading-edge slat and the main wing while the leading-edge slat is retracted. However, it has been found that the vortex can sometimes press a flexible separating surface against the rear face of the leading-edge slat when the latter is in the extended state, so that it does not automatically position itself on the separation flow line owing to this relationship of the forces.
It is thus more advantageous for the separating surface to be formed by an inflatable member or balloon which is fitted on the additional airfoil and to which pressure can be applied. When the additional airfoil is extended, the member can be inflated by applying pressure so that it completely replaces the vortex flow region. As the leading-edge slat is being retracted, the pressure is allowed out, and the member is pressed into the cove between the leading-edge slat and the main wing.
Additional objects, features and advantages of the invention will be set forth in the description of preferred embodiments which follows.